For the separation of suspended solids from a fluid by filtration, two basic configurations are used, direct-flow filtration (also called normal flow filtration) and crossflow filtration (also called tangential flow filtration).
In direct-flow filtration, fluid is introduced perpendicularly to the filter surface and then passes directly through the filter.
In crossflow filtration, on the other hand, a fluid flow is passed tangentially along the filter surface. Particles smaller than the pore size of the filter pass through the membrane as a permeate (filtrate), while everything else is retained on the feed side of the membrane as a retentate. Usually the fluid flow is recirculated across the filter surface. Since the retained products are swept along the surface by the tangential flow and do not build up at the filter surface as in direct-flow filtration, a crossflow filter can operate continuously at relatively high solids loads without being blocked by solids material.
Depending on the pore size of the filter, typically a membrane, crossflow filtration is classified as either a microfiltration (MF) or ultrafiltration (UF) process. Filter membrane configurations include flat-sheet membranes, which are usually used in plate and frame-type cassettes, spiral-wound module formats and hollow fibre modules. A filter cassette typically comprises a stacked assembly of porous membrane components and filtrate and retentate flow screen components. In a filtration module, several such filter cassettes are usually used.
A crossflow filtration system typically includes a filtration module, a feed tank, a pump for feeding liquid from the feed tank to the filtration module via a feed line, a return line for circulating retentate back to the feed tank, a valve in the retentate return line for applying pressure, and a permeate line for removing permeate from the filtration module.
Performing non-process activities on such a system, such as during installation, functional testing or simulation of different methods is today used either with the normal filtration module in place or with the system components disassembled and with tubes and external valves connected. In the former case, some process parameters, including back pressure from feed to retentate and flow rate from filtration module to permeate line can not be varied. There is also a risk of damaging the filter membrane assembly. The alternative of using disassembly of system components, on the other hand, is rather complicated and requires substantial additional space.
U.S. Pat. No. 4,846,970 discloses a crossflow filtration membrane test unit which can be used to test the performance and flow characteristics of filtration membranes. The device includes means for closely simulating the flow dynamics existing in a full-size crossflow filtration system.
It is an object of the present invention to provide means for simplifying and improving functional testing and other non-process activities on an operational full-size crossflow filtration system rather than testing filter membrane samples as in the above-mentioned U.S. Pat. No. 4,846,970.